bone conduction

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There are times when being seen to listen to music through headphones might get you into trouble. For these moments, reach for a handy solution: bone conduction speakers that discreetly pipe the music to your eardrums through the bone of your skull. [Samuel] wanted just such a covert music listening device, so created his own in a set of 3D-printed glasses.

He first tried using an Adafruit bone-conducting transducer but found that to be too bulky. What you see here is a smaller module that [Samuel] found on AliExpress (search for bone conduction module). The GD-02 is much smaller and thus more suitable for hiding in the arm of a pair of glasses. For the rest of the electronics he used a PCB and battery from a donated set of broken Bluetooth headphones, a space for which he was able to conceal easily in the 3D-printed frame of the glasses. The battery is in one arm and the board in the other, and he says the wiring was extremely fiddly.

The result is a surprisingly svelte set of specs that you might not immediately think concealed some electronics. His choice of bright yellow filament might give the game away, but overall he’s done a great job. This certainly isn’t the first bone conduction project we’ve shown you, some of the others have used motors instead of bone conduction transducers.

[Matlek] had an interesting problem. On one hand, a 40 minute bike commute without music is a dull event but in France it is illegal for any driver to wear headphones. What to do? Wanting neither to break the law nor accept the risk of blocking out surrounding sounds by wearing headphones anyway, and unwilling to create noise pollution for others with a speaker system, [Matlek] decided to improvise a custom attachment for a bike helmet that plays audio via bone conduction. We’ll admit that our first thought was a worrisome idea of sandwiching metal surface transducers between a helmet and one’s skull (and being one crash away from the helmet embedding said transducers…) but happily [Matlek]’s creation is nothing of the sort.

A 3D printed rack and pinon provides adjustability and stable contact with the “sweet spot” behind each ear.

The bone conduction is cleverly achieved by driving small DC motors with an audio signal through a TPA2012 based audio amplifier, which is powered by a single 18650 cell. By using motors in place of speakers, and using a 3D printed enclosure to hold the motors up to a sweet spot just behind the ears, it’s possible to play music that only the wearer can hear and does not block environmental sounds.

[Matlek] didn’t just throw this together, either. This design was the result of researching bone conduction audio, gathering a variety of different components to use as transducers, testing which performed best, and testing different locations on the body. Just behind the ear was the sweet spot, with the bony area having good accessibility to a helmet-mounted solution. Amusingly, due to the contact between the motors and the rest of the hardware, the helmet itself acts as a large (but weak) speaker and faint music is audible from close range. [Matlek] plans to isolate the motors from the rest of the assembly to prevent this.

Recent news reports have claimed that an MIT headset can read your mind, but as it turns out that’s a little bit of fake news. There is a headset — called AlterEgo — but it doesn’t actually read your mind. Rather, it measures subtle cues of you silently vocalizing words. We aren’t sure exactly how that works, but the FAQ claims it is similar to how you experience reading as a child.

If you read much science fiction, you probably recognize this as subvocalization, which has been under study by the Army and NASA. However, from what we know, the positioning of sensor electrodes is crucial and can vary not only by speaker, but also change for the same speaker. Perhaps the MIT device has found a way around that problem. You can see a video of the system, below.

The victim donor hardware for this project is a toothbrush meant for kids called Tooth Tunes. They’ve been around for years, but unless you’re a kid (or a parent of one) you’ve never heard of them. That’s because they generally play the saccharine sounds of Hannah Montana and the Jonas Brothers which make adults choose cavities over dental health. However, we’re inclined to brush the enamel right off of our teeth if we can listen to The Amp Hour, Embedded FM, or the Spark Gap while doing so. Yes, we’re advocating for a bone-conducting, podcasting toothbrush.

[Joe’s] hack starts by cracking open the neck of the brush to cut the wires going to a transducer behind the brushes (his first attempt is ugly but the final process is clean and minimal). This allows him to pull out the guts from the sealed battery compartment in the handle. In true [Grand] fashion he rolled a replacement PCB that fits in the original footprint, adding an SD card and replacing the original microcontroller with an ATtiny85. He goes the extra mile of making this hack a polished work by also designing in an On/Off controller (MAX16054) which delivers the tiny standby current needed to prevent the batteries from going flat in the medicine cabinet.

Check out his video showcasing the hack below. You don’t get an audio demo because you have to press the thing against the bones in your skull to hear it. The OEM meant for this to press against your teeth, but now we want to play with them for our own hacks. Baseball cap headphones via bone conduction? Maybe.

Update: [Joe] wrote in to tell us he published a demonstration of the audio. It uses a metal box as a sounding chamber in place of the bones in our head.

There are many ways to take an electrical audio signal and turn it into something you can hear. Moving coil speakers, plasma domes, electrostatic speakers, piezo horns, the list goes on. Last week at the Electromagnetic Field festival in the UK, we encountered another we hadn’t experienced directly before. Bite on a brass rod (sheathed in a drinking straw for hygiene), hear music.

The TOG Skull Radio demo box

This was Skull Radio, a bone conduction speaker courtesy of [Tdr], one of our friends from TOG hackerspace in Dublin, and its simplicity hid a rather surprising performance. A small DC motor has its shaft connected to a piece of rod, and a small audio power amplifier drives the motor. Nothing is audible until you bite on the rod, and then you can hear the music. The bones of your skull are conducting it directly to your inner ear, without an airborne sound wave in sight.

The resulting experience is a sonic cathedral from lips of etherial sibilance, a wider soft palate soundstage broadened by a tongue of bass and masticated by a driving treble overlaid with a toothy resonance before spitting out a dynamic oral texture. You’ll go back to your hi-fi after listening to [Tdr]’s Skull Radio, but you’ll know you’ll never equal its unique sound.

(If you are not the kind of audiophile who spends $1000 on a USB cable, the last paragraph means you bite on it, you hear music, and it sounds not quite as bad as you might expect.)

Biometrics–the technique of using something unique about your body as a security device–promises to improve safety while being more convenient than a password. Fingerprints, retinal scans, and voice identification have all found some use, although not without limitations.

Now researchers in Germany want you to use your head, literally. SkullConduct measures vibrations of your skull in response to a sonic signal. A small prototype was successful and is particularly well suited for something you are holding up to your head anyway, like a smartphone or a headset like a Google Glass.

There are some limitations, though. For one thing, background noise can be a factor. It stands to reason, also, that more testing is necessary.

When a hacker finds himself with a metal disc and magnet surgically implanted in his skull, chances are pretty good that something interesting will come from it. [Eric Cherry]’s implant, designed to anchor a bone-conduction hear aid, turned out to be a great place to mount a low-cost Bluetooth speaker for his phone – at least when he’s not storing paperclips behind his ear.

With single-sided deafness, [Eric]’s implant allows him to attach his bone-anchored hearing aid (BAHA), which actually uses the skull itself as a resonator to bypass the outer ear canal and the bones of the middle ear and send vibrations directly to the cochlea. As you can imagine, a BAHA device is a pretty pricey bit of gear, and being held on by just a magnet can be tense in some situations. [Eric] decided to hack a tiny Bluetooth speaker to attach to his implant and see if it would work with his phone. A quick teardown and replacement of the stock speaker with a bone-conduction transducer from Adafruit took care of the electronics, which were installed in a 3D printed enclosure compatible with the implant. After pairing with his phone he found that sound quality was more than good enough to enjoy music without risking his implant. And all for only $22 out-of-pocket. While only a Bluetooth speaker in its current form, we can see how the microphone in the speakerphone might be used to build a complete hearing aid on the cheap.

We think this is a great hack that really opens up some possibilities for the hearing impaired. Of course it’s not suitable for all types of hearing loss; for more traditional hearing aid users, this Bluetooth-enabled adapter might be a better choice for listening to music.